1. Field of the Invention
The invention relates to a process for electroplating precious metals such as a gold and alloys.
2. Description of the Prior Art
Precious metal electroplating processes such as gold electroplating processes have many important industrial applications, including production of jewelry, optical devices, as well as the production of electronic circuits and components. To simplify discussion, gold electroplating will be used as a concrete example. However, the discussion applies equally well to other precious metals such as rhodium, palladium, iridium and platinum. In recent years, the use of gold in such industrial applications has been increasing very rapidly. There are several advantages to the use of gold. First of all, it does not form a surface insulating film such as an oxide film. For this reason, gold has a high surface luster which is quite attractive when used in jewelry articles. For the same reason, the optical reflection properties of gold are attractive, which makes its use in optical devices highly desirable. Again, for the same reason, its use in electric circuits and components is highly desirable because surface contact to gold usually has low electrical resistance.
Gold also has the advantage of being chemically inert. This is due to the fact that no surface insulating layer is formed on gold. The use of gold often increases the lifetime and reliability of devices and articles since gold is not affected by many chemicals and adverse conditions of temperature and humidity. A particular case in point is the production of integrated electronic circuits. Here quite thin and narrow conducting paths are required. Many metals (i.e., copper) might be satisfactory as far as electrical conductivity is concerned, but they rapidly degrade with time. Gold has the advantage of being inert, and also has quite high electrical conductivity.
Because of its excellent electrical contact properties, gold is often used in electrical connectors, switches and relays. By the addition of small amounts of various elements (for example, arsenic, cobalt, nickel), gold can be made quite hard and resistant to abrasion. For the above reasons, and because of the extensive growth of the integrated circuit industry, the industrial use of gold has increased tremendously in recent years. This fact, together with the high and increasing cost of gold, has made it highly desirable to use gold in as efficient a manner as possible. Particularly significant is the development of processes which deposit gold only in the particular areas where it is required, and not in areas where it is not required. This type of process is referred to here as selective gold electroplating.
Several processes have been described for limiting the area of electroplating. For example, in U.S. Pat. No. 3,051,632 issued Aug. 28, 1962 to M. A. Dike, a copper electroplating process is described for building up the thickness of a copper tubing elbow. In this procedure only the area where copper electroplating is desired is immersed in the electroplating solution. This procedure may be quite satisfactory for building up the thickness of tubes, but is usually not applicable to gold plating intricate electronic components. Procedures are also known for producing pipe couplings by electrodeposition on a core (see, for example, U.S. Pat. No. 2,181,257 issued Nov. 28, 1939 to A. C. Arbogast), but again such a procedure is not suitable for electroplating gold on various devices. Other references having to do with electroplating are U.S. Pat. No. 2,229,077 issued Jan. 21, 1941 to W. H. Hammond et al., and 2,358,104 issued Sep. 12, 1944 to A. C. Scavullo. Plating of gun barrels is described in U.S. Pat. No. 2,780,019 issued Feb. 5, 1957 to G. C. Sullivan. A procedure for electroforming hollow bodies is described in U.S. Pat. No. 2,879,209 issued Mar. 24, 1959 to Samuel Fialkoff.